1. Field of the Invention
This invention relates to an optical isolator independent of a direction of polarization and, more particularly, a polarization independent optical isolator which is independent of the direction of polarization and very easy to carry out assemblage and alignment.
2. Description of the Prior Art
A semiconductor laser is very common as a coherent light source for an applied optical instrument, light communication equipment or the like. However, the semiconductor laser has a serious problem or disadvantage such that, when a coherent light transmitted from the semiconductor laser is directed to an optical system, such as to one end surface of a connector, the coherent light reflected back to the semiconductor laser light source causes the laser oscillation to become unstable.
To eliminate the problems encountered by the semiconductor laser, an optical isolator has been provided at an output-side of the semiconductor laser (in this specification, it is assumed that the output-side of the laser source is always located at the left side of drawings), and a reflected laser light has been prevented from returning back to the semiconductor laser light source by designing and arranging forward and reverse optical paths properly within the optical isolator.
The optical isolator of the prior art is provided by an optical system including a magneto-optical element for separating a reflected laser light (shown by a light beam "b" propagating from right to left in the drawings) from a laser light in forward direction transmitted by the semiconductor laser light source (shown by a light beam "a" propagating from left to right) based on the Faraday rotation effect.
Generally, the optical isolator of the prior art includes a magneto-optical element 3 (a Faraday rotation element) arranged at the inside of a permanent magnet 4 which is placed between a polarizer 1 and an analyzer 2, as shown in FIG. 1, for intercepting the reflected laser light or returning laser light coming back to the semiconductor laser light source.
More specifically, in FIG. 1, the laser light "a" transmitted, in forward direction, from the semiconductor laser light source passes through the magneto-optical element 3 after being converted, at the polarizer 1, into a linearly polarized laser light having a plane of oscillation in a vertical direction. A polarization plane of the incident laser light to the magneto-optical element 3 is rotated clockwise by an amount of 45 degrees when it is viewed from the side of the semiconductor laser light source, whereas the direction of rotation of the polarization plane may depend on a direction of magnetic force of the permanent magnet and/or a material of the magneto-optical element.
For simplifying and clarifying the description, it is assumed for the direction of polarization rotation that, when it is viewed from the side of the semiconductor laser light source, a right-handed rotation is always designated by a clockwise rotation while a left-handed rotation is designated by a counter-clockwise rotation unless otherwise specified hereinafter.
The analyzer 2 is arranged in perpendicular to a plane of polarization the polarized wave cut-off direction of which is rotated clockwise by 45 degrees. Accordingly, a polarized component of the ordinary light "a" having a polarization plane in a vertical direction being transmitted from the semiconductor laser light source can transmit through all the optical elements, such as the polarizer 1, magneto-optical element 3 and analyzer 2, without any loss except a little absorption and Fresnel reflection.
On the other hand, the laser light "b" in reverse direction, or the reflected laser light (reflected returning light) in return to the semiconductor laser light source, enters the magneto-optical element 3 after passing through the analyzer 2. A polarization plane of the reflected laser light "b" in reverse direction incident to the magneto-optical element 3 is in turn rotated by 45 degrees in the same manner as in the rotation of the laser light "a" in forward direction.
Since the rotation of the polarization plane of the reflected laser light "b" in reverse direction is also performed toward the same direction as that of the laser light "a" in forward direction regardless of the direction of propagation due to a specific feature of the magneto-optical element or the non-reciprocal effect, the polarization plane of the laser light "b" is again rotated clockwise by 45 degrees at the magneto-optical element 3. Therefore, the polarization direction of the reflected laser light "b" in reverse direction after passing through the magneto-optical element 3 has an angle of 90 degrees in total to the polarization direction of the laser light "a" in forward direction.
In this manner, the reflected laser light "b" in reverse direction that has passed through the magneto-optical element 3, or the reflected light (reflected returning light) to the semiconductor laser light source, is unable to pass through the polarizer 1 and is prevented by the polarizer 1 from returning to the semiconductor laser light source.
An optical isolator fundamentally has a function of allowing to pass through an incident light from the side of semiconductor laser light source (left side), for example, an output laser light, while intercepting an incident light from the counter side (right side), for example, a reflected laser light in reverse direction.
The foregoing description as to the function of the optical isolator is directed to the one employing a dichroic polarizer in both the polarizer 1 and analyzer 2, for example, the dichroic polarizer produced by Corning Glass Inc. and known under the trade name of "Polarcor", however, it is also possible to attain substantially the same function as that of the above by employing a birefringent crystal plate such as of rutile single crystal for both the polarizer and analyzer. The difference between these two types of optical isolators may be found in such that the reflected laser light "b" is intercepted at the polarizer 1 in case of the dichroic polarizer, as described hereinabove.
On the contrary, in case of the birefringent crystal plate, the reflected laser light "b" is prevented from returning back to an emitting point of the semiconductor laser light source by altering an optical path of the reflected laser light "b" diagonally within the birefringent crystal plate, wherein the optical path or optical axis of the reflected laser light "b" is shifted from that of the laser light "a" for being emitted from the birefringent crystal plate toward a point where is completely different from the optical path of the laser light "a" in forward direction.
The emitted semiconductor laser light is substantially a linearly polarized light, so that the optical isolator can transmit the laser light therethrough, substantially with no loss, by aligning a direction of polarization of the laser light with the polarized light transmissible direction of the polarizer. However, if the optical isolator shown in FIG. 1 were inserted between optical fibers wherein substantially non-polarized light rays are propagating, all the light rays, polarization planes of which are not identical to the direction of polarization of the polarizer, will be impeded and blocked by the polarizer 1.
In general, an amount of light loss at the polarizer causing from the blockage or isolation may reach to an order of 3 dB. Several optical isolators have been proposed in the past for eliminating the light loss encountered by the insertion of the optical isolators and some of them are disclosed in Japanese Patent Publications.
In Japanese Patent Publications No. 60-51690 and No. 58-28561, three birefringent crystal plates are assembled to provide the optical isolator, while in a Japanese Patent Publication No. 60-49297, two birefringent crystal plates are combined with an optically active element, and further in a Japanese Patent Publication No. 61-58809, tapered birefringent crystal plates and lenses are employed.
According to the optical isolators disclosed in the Japanese Patent Publications above, the light having omnidirectional polarization is split up once into two orthogonal polarized wave components by means of a birefringent crystal plate, however, since these orthogonal polarized wave components are combined again by means of another birefringent crystal plate and/or a lens, the both polarized wave components can be transmitted through the optical isolator without losing any.
On the contrary, the reflected light in reverse direction is, owing to the non-reciprocity of the magneto-optical element, guided out of the magneto-optical element from a point other than a point at where the light in forward direction has entered, thus the reflected light in reverse direction never returns back to the semiconductor laser light source or semiconductor laser light emitting point.
Further, a polarization independent optical isolator has also been proposed in a Japanese Patent Publication No. 60-51690, a configuration of which is shown in FIGS. 2 and 4. FIG. 2 is a side view of the polarization independent optical isolator showing optical paths of the laser light "a" propagating therethrough, while FIG. 4 is a side view of the polarization independent optical isolator showing optical paths of the reflected laser light "b" propagating in reverse direction through the optical isolator.
In FIGS. 2 and 4, element 5 designates a first birefringent crystal plate provided by cutting an uniaxial crystal, such as a rutile single crystal and the like, into a plate with parallel surfaces in such a manner as an optical axis of the uniaxial crystal being inclined against the parallel surfaces, and element 6 designates a magneto-optical element made of, for example, a bismuth substituted iron garnet single crystal having a Faraday rotation angle of 45 degrees.
Further, element 7 designates a second birefringent crystal plate an optical axis of which is inclined by the same amount as that of the first birefringent crystal plate 5 against the surfaces thereof but rotated clockwise by an amount of 45 degrees from the first birefringent crystal plate 5 about the incident laser light "a" as an axis, element 8 designates a third birefringent plate an optical axis of which is inclined by the same amount as that of the first birefringent crystal plate 5 against the surfaces thereof but rotated counter-clockwise by an amount of 45 degrees from the first birefringent crystal plate 5 about the incident laser light "a" as an axis, and element 9 designates a permanent magnet to saturate the magneto-optical element 6 magnetically.
Positions of light exit and directions of polarization at surfaces of the first birefringent crystal plate 5, magneto-optical element 6, the second birefringent crystal plate and third birefringent crystal plate 8 are illustrated in FIG. 3 and that directions of optical axis of the birefringent crystal plates 5, 7 and 8 are also illustrated in the same Figure.
The operation of the optical isolator shown in FIGS. 2 and 4 will now be described in more detail. As shown in FIG. 2, the incident laser light "a" in forward direction is split into two laser beams having orthogonal oscillation planes, or an ordinary light and extraordinary light, by means of the first birefringent plate 5. The ordinary light proceeds directly through the first birefringent crystal plate 5 while the extraordinary light proceeds diagonally through the same birefringent crystal plate 5.
The ordinary light and extraordinary light then enter the magneto-optical element 6 after passing through the first birefringent crystal plate 5 and propagating along parallel optical paths, each polarization plane of which is in turn rotated clockwise by 45 degrees at the magneto-optical element 6. The ordinary light and extraordinary light then enter into the second birefringent crystal plate 7 after passing through the magneto-optical element 6.
The second birefringent crystal plate 7 is so arranged that an optical axis of which is inclined by an amount of 45 degrees against the optical axis of the first birefringent crystal 5. Accordingly, only the polarized component of the incident laser light to the second birefringent crystal plate 7 in parallel with the optical axis thereof proceeds diagonally within the second birefringent crystal plate 7. The laser light passed through the second birefringent crystal plate 7 then enter into the third birefringent crystal plate 8. The third birefringent crystal plate 8 is so arranged that its optical axis inclines 90 degrees against the optical axis of the second birefringent crystal 7. Accordingly, the polarized component of the incident laser light in parallel with the optical axis of the third birefringent crystal plate 8 proceeds diagonally therethrough.
By selecting the thickness of the second birefringent crystal plate 7 and third birefringent crystal plate 8 to become one by square root (1/.sqroot.2) of the thickness of the first birefringent crystal plate 5, it is possible to combine the two laser light beams separated at the first birefringent crystal plate 5 into one laser light beam at the third birefringent crystal plate 8.
On the other hand, the reflected laser light "b" in reverse direction returns back to the magneto-optical element 6 by passing through the third birefringent crystal plate 8 and second birefringent crystal plate 7, as shown in FIG. 4, on the track of the same optical path as that of the laser light "a" in forward direction. A direction of polarization of the reflected laser light "b" in reverse direction that has passed through the magneto-optical element 6 is orthogonal to the direction of polarization of the laser light "a" in forward direction as it has rotated clockwise by the amount of 45 degrees at the magneto-optical element 6.
Consequently, the reflected laser light "b" in reverse direction will be led out of the first birefringent crystal plate 5 at a point other than the point of incidence of the laser light "a" in forward direction upon passing through the first birefringent crystal plate 5.
In accordance with the polarization independent optical isolator, as described above, the non-polarized laser light designated by "a" propagating from the left side or the side where the semiconductor laser light source is located (laser light emitter side) and the non-polarized laser light "b" propagating from the right side or the other side of the optical isolator can be isolated completely.
Another system has been proposed by Matsumoto in a Japanese Patent Publication No. 58-28561, wherein lenses 10 and 11 are provided at the both outer limits of the first birefringent crystal plate 5 and the third birefringent crystal plate 8, as shown in FIG. 5, for converging the laser light within the optical isolator. In accordance with this configuration, a distance for separating the two polarized laser components within the optical isolator can be shortened and the thickness of the birefringent crystal plates can be decreased. As seen in the optical isolator of FIG. 5, optical paths of the laser light "a" in forward direction and laser light "b" in reverse direction are the same as that of FIGS. 2 and 4.
Another type of optical isolator has been proposed by Uchida in a Japanese Patent Publication No. 60-49297, wherein birefringent crystal plates and an optically active element are employed as shown in FIG. 6. This type of optical isolator is provided by substituting an optically active element 13 for the second birefringent crystal plate 7 of the polarization independent optical isolator of FIG. 4.
In accordance with this optical isolator of FIG. 6, the polarization plane of the laser light in forward direction incident from the left side of the birefringent crystal plate 5 is rotated clockwise by 45 degrees at the magneto-optical element 6, however, the polarization plane of the laser light is once again rotated counter-clockwise by 45 degrees at the optically active element 13. Thus, the incident laser light to the birefringent crystal element 12 has the same polarization plane as that of the laser light passed through the birefringent crystal plate 5 or the incident laser light to the magneto-optical element 6. In this way, the ordinary light and extraordinary light which have been split into two beams at the birefringent crystal plate 5 are recombined by means of the birefringent crystal plate 12.
In contrast with the above, in case of the reflected laser light in reverse direction or the light propagating from the right side of the birefringent crystal plate 12 of the optical isolator to the left, the plane of polarization is rotated clockwise by 45 degrees when passing through the optically active element 13. The rotated polarization plane of the reflected laser light is again rotated clockwise by 45 degrees when passing through the magneto-optical element 6.
Accordingly, the polarization plane of the reflected laser light in reverse direction that has passed through the magneto-optical element 6 has a difference of 90 degrees with the polarization direction of the laser light in forward direction. Therefore, the reflected laser light in reverse direction entered into the birefringent crystal plate 5 will come out therefrom at a point other than the incident point of the laser light in forward direction, thus the reflected laser light in reverse direction is prevented from returning back to the point of emitting laser light or the semiconductor laser light source.
In addition to the above, still another type of optical isolator has been proposed by Shirasaki in a Japanese Patent Publication No. 61-58809, wherein tapered birefringent crystal plates are employed (as shown in FIG. 8). This type of optical isolator employs tapered birefringent crystal plates 14 and 15 as the birefringent crystal plates.
In accordance with the optical isolator of FIG. 8, the laser light "a" propagating in forward direction enters into the second tapered birefringent crystal plate 15, transmits therethrough as being separated in parallel and enters into a lens 11 for being focused onto an optical fiber 17 at the receiving side.
On the contrary, as shown in FIG. 8, the reflected laser light "b" in reverse direction enters into the first birefringent crystal plate 14 after passing through the magneto-optical element 6. The optical path of the reflected light "b" is then diverged by the act of the first birefringent crystal plate 14, thus the reflected laser light "b" never reaches to the optical fiber 16 at the transmitter side.
Recently, an optical fiber communication attracts attention in a communication area as a high speed and large capacity communication system. In the light of a tendency of the above, many researches and developments have been made in the past for materializing the optical fiber communication, putting the optical fiber communication into a practical use and obtaining a higher speed in the optical fiber communication. Accordingly, there have been proposed various types of optical isolators such as described herein above which constitute one of the main parts of a transmitter and receiver in the optical fiber communication system.
However, all the proposed polarization independent optical isolators employ the Faraday rotation effect of the magneto-optical element, therefore, a misalignment of an optical system and errors, such as a deviation from the Faraday rotation angle of 45 degrees, to be encountered in a process of producing the an optical element or assembling an optical device have resulted in serious problems heretofore.
More specifically, a bismuth substituted iron garnet single crystal produced by a liquid phase epitaxial method is normally employed as the magneto-optical element for the optical isolators. The bismuth substituted iron garnet single crystal grown on a non-magnetic garnet substrate to a thickness of several hundred microns through the liquid phase epitaxial method is ground precisely to the thickness with which the Faraday rotation angle of 45 degrees can be attained.
The bismuth substituted iron garnet single crystal usable for the magneto-optical element of the optical isolator is selected from a number of pellets obtained by grinding the bismuth substituted iron garnet single crystal based on an allowable thickness tolerance. The thickness of the pellets of the bismuth substituted iron garnet single crystal selected for the magneto-optical element has variations of several microns. Since the thickness variations of the bismuth substituted iron garnet single crystal are caused solely from an accuracy of grinding operation, it is impossible to eliminate the thickness variations completely in accordance with the present grinding technical level. Accordingly, heretofore, the quality and optical accuracy of the bismuth substituted iron garnet for use in the optical isolator as the magneto-optical element can only be maintained by the selecting method as described above, whereas the quality of the bismuth substituted iron garnet can be improved by decreasing the tolerances while the yield rate of the products is lowered, and it becomes unprofitable.
Further, it is known that an amount of a solid solution of bismuth in the bismuth substituted iron garnet single crystal provided by the liquid phase epitaxial method may vary in response to a slight change in a condition of growth of the single crystal, and a Faraday rotation angle per unit of thickness of the grown-up single crystal may vary in response to the amount of the solid solution of bismuth in the single crystal. The quality, or the tolerance of the Faraday rotation angle, of the bismuth substituted iron garnet single crystal, presently available on the market, for use as a magneto-optical is normally 1-2%.
Accordingly, the magneto-optical element obtained by grinding the bismuth substituted iron garnet single crystal produced by the liquid phase epitaxial method has an error of 0.5-1.0% in the Faraday rotation angle.
As described above, the bismuth substituted iron garnet single crystal to be utilized as the magneto-optical element normally has such error in the Faraday rotation angle as to be equivalent, at least, to the tolerance of the selection. Therefore, to attain a high isolation as an optical isolator, it is necessary to adjust or compensate a direction of the optical axis of the first birefringent plate by an amount which is commensurate with an angle of deviation (.DELTA..theta.) from the reference angle of 45 degrees.
If it is assumed that the deviation angle .DELTA..theta. is one degree (.DELTA..theta.=1.degree.) from the reference angle, a extinction ratio of the first birefringent plate will be 35 dB (the theoretical maximum extinction ratio) in accordance with an equation of -10.multidot.log[sin.sup.2 (.DELTA..theta.)]. Practically, the isolation required for the optical isolator is at or above 30 dB. Therefore, if the deviation angle of .DELTA..theta. from the reference angle is one degree, a required performance can be satisfied theoretically, and no adjustment as well as compensation will be required for the birefringent crystal plate along the optical axis thereof.
However, in the actual state, the deviation angle of .DELTA..theta. from the reference angle will be expanded from the theoretical value of one degree owing to the temperature dependence and light wave length dependence of the Faraday rotation angle of the magneto-optical element and further to a difference between a wave length of laser light used in the process of assembling and a wave length of laser light in the actual use, which difference is normally several nm, thus resulting in the difficulty to maintain the utmost of 30 dB.
As to an optical isolator, for example, having the deviation angle .DELTA..theta. of one degree assembled by employing Ho.sub.1.1 Tb.sub.0.6 Bi.sub.1.3 Fe.sub.5 O.sub.12 available on the market, if an environmental temperature varies more than 12.degree. C. or the wave length of the laser light in use differs by 6 nm or more from that of the laser light utilized in the process of assembling, the deviation angle .DELTA..theta. from the reference angle will become 1.8 degrees or more, thus decreasing the isolation to less than 30 dB and losing practicality.
To ensure the practical performance and quality of the optical isolator, it is necessary for optical isolation to maintain at least 40 dB or more during the process of assembling. However, it is practically impossible to maintain such high optical isolation only by improving the quality of the magneto-optical element and all that it is economically disadvantageous. The adjustment and compensation along the optical axis of the first birefringent crystal plate are prerequisite as the second best plan to improve the optical isolation, and further they have the importance as the fundamental technique in the industrial practice.
An outline of the adjustment and compensation along the optical axis of the first birefringent crystal plate will now be described by referring to FIG. 7.
FIG. 7 is a diagram showing an optical isolator for illustrating the adjustment and compensation along the optical axis of the optical isolator, for example, by referring to the optical isolator shown in FIGS. 2 and 4. The operation of adjustment and compensation of the optical isolator is provided by:
1) mounting lenses 10 and 11 and optical fibers 16 and 17 at the both ends of the optical isolator;
2) transmitting the laser light from the optical fiber 16 at the left side or the side of semiconductor laser light source and confirming the correct reception of the laser light by the optical fiber 17 at the right side or the receiving side;
3) transmitting the laser light in a reverse direction from the optical fiber 17 at the right side and rotating the first birefringent crystal plate 5 to minimize the strength of the laser light "b'" (shown by a dot line in FIG. 7) reaching to the optical fiber 16 at the left side.
In general, a core diameter of the optical fiber is so small such as of 5-10 .mu.m. Accordingly, the operation for adjustment and compensation along the optical axis of the optical isolator is implemented precisely with extreme care. However, an optical path or optical axis of the optical system is displace easily by the adjustment of the first birefringent crystal plate 5, whereas if the axis of optical path of the optical system is displaced, the laser light "b'" can never be received by the optical fiber 16.
Normally, the adjustment, compensation and control of the optical axis of the first birefringent crystal plate 5 and that of the axis of optical path of the optical system are implemented by tracing the laser light "b'" with the optical fiber 16 coupled to a power meter. However, in an actual operation, it is often hard for the operator to discriminate, or decide, whether or not a state of vanishment is resulted from the proper alignment of the optical path of the birefringent crystal plate, since the same vanishment may happen when no laser light reaches to the optical fiber due to the misalignment of the axis of optical path of the optical system. Under the present technical level, the optical adjustment of the birefringent crystal plate 5 has to be implemented by moving the optical fibers 16 precisely with use of a precision locating device as tracing scrupulously, with extreme care, the incident laser light "b'" to the optical fiber 16. Hence, it is still difficult to mechanize and adopt a mass production system. The drawback as described above has been one of the main reasons for delaying the versatility of the polarization independent optical isolator.
For making easier the adjustment and compensation of the optical axis of the first birefringent crystal plate and that of the axis of optical path of the optical system, a separation between the laser light "b" and the laser light "b'" may be increased. In accordance with the method as stated above, it is possible to accept or receive the laser light "b'" by making use of a photo-detector which is available on the market, if the separation between the laser light "b" and the laser light "b'" is selected to be several millimeters. Consequently, the operation of adjustment and compensation of the optical axis of the first birefringent crystal plate and that of the axis of optical path of the optical system become very easy.
However, in order to provide, for example, a separation of several millimeters (mm) between the light "b" and light "b'", it is required to select the thickness of the birefringent crystal plate 5 to be several centimeters or more and also to thicken other birefringent crystal plates, such as the birefringent crystal plate 7 and birefringent crystal plate 8 as well. Normally, the birefringent crystal plates are made of expensive rutile single crystal. Therefore, the method of increasing the separation between the laser light "b" and laser light "b'" lacks rationality and inevitability in the light of economical and technical point of view. Thus, it is no exaggeration to say that the method described above lacks practicality as an industrial technology.
The operation of adjustment and compensation of the optical axis of the first birefringent crystal plate and that of the axis of optical path of the optical system has been described hereinbefore by referring to FIGS. 2 and 4, however, optical isolators shown in FIGS. 5 and 6 also have the similar problems.
Now referring to FIG. 8, there is shown an optical isolator having tapered birefringent crystal plates 14 and 15, wherein a separation angle of the laser light "a" and laser light "b" can be widened by increasing taper angles of the tapered birefringent crystal plates 14 and 15. Consequently, the operation of adjustment and compensation of the optical axis of the first birefringent crystal plate and that of the axis of optical path of the optical system become somewhat easier as compared with the other prior-art methods. However, if the taper angles are widened, there is provided a large-sized optical isolator owing to the fact that a distance between the lens 11, which is for converging the light transmitted through the tapered birefringent crystal plate 15 into the optical fiber 17, and the optical fiber 17 is elongated since the separation between the light beams in the forward direction becomes large, and there cause problems such as increasing an optical coupling loss and the like.
As described above, in accordance with the prior-art polarization independent optical isolator, since a laser light beam or laser light emitted from the semiconductor laser light source is reflected at surfaces of the optical system, there is a reflected return laser light to the semiconductor laser light source. The laser oscillation at the semiconductor laser light source becomes unstable if the reflected return laser light reenters therein. Accordingly, in order to make the optical isolator practical in use, it has been inevitable to implement an angular adjustment of the birefringent crystal plate, or the operation of the optical adjustment and compensation in a direction along the optical axis. As it has been described hereinbefore, the operation of the optical adjustment and compensation of the optical isolator along the optical axis is extremely difficult. Therefore, one of the most important theme in the field of optical fiber communication is to provide an optical isolator which only requires easy or no optical adjustment and compensation for popularizing the semiconductor laser and, more particularly, the optical fiber communication system
To eliminate such difficulties as described above, a polarization independent optical isolator, shown in FIG. 9, has been proposed by Shiraishi and Kawakami, Research Institute of Electrical Communication, Tohoku University (Trans. IECE Japan, Spring 1991, C-290).
In FIG. 9, elements 18 and 19 are birefringent crystal plates, elements 20 and 21 are half-wave plates, and elements 22 and 23 are polarization dependent optical isolators. An incident laser light "a" is split into crossed laser beams or an ordinary light "c" and extraordinary light "d" at the first birefringent crystal plate 18, whereas an optical path for the ordinary light "c" is indicated by the same character "c" and that of the extraordinary light "d" by the same character "d". A polarization plane of the extraordinary light "d" is rotated by 90 degrees through the half-wave plate 20 inserted in the optical path thereof to coincide with the polarization plane of the ordinary light "c". The extraordinary light "d" then enters the polarization dependent optical isolators 22 and 23 together with the ordinary light "c". Since the incident extraordinary light "d" has the same polarization plane as that of the ordinary light "c", the extraordinary light "d" is able to pass through the polarization dependent optical isolators 22 and 23 both of which are arranged in alignment with the polarization direction of the ordinary light "c". The polarization planes of the ordinary light "c" and extraordinary light "d" are rotated by 90 degrees when passing through the polarization dependent optical isolators 22 and 23, hence each of the light "c" and light "d" becomes the extraordinary light. The light "c" is in turn rotated by 90 degrees again by means of another half wave plate 21 inserted in the optical path of the light "c". Accordingly, the ordinary light "c" and extraordinary light "d" are then recombined by the second birefringent crystal plate 19.
On the other hand, the reflected laser light "b" in reverse direction is not in a position to pass through the polarization dependent optical isolators 22 and 23. Therefore, an adjustment of the polarizer to compensate an optical misalignment caused by an angular deviation of the Faraday rotator can be implemented satisfactorily only by aligning the polarization dependent optical isolators beforehand. Accordingly, the assembling and adjusting of this type of optical isolator are comparatively easier than that of the firstly mentioned conventional prior-art optical isolators. However, this optical isolator requires two birefringent crystal plates, two half-wave plates and two polarization dependent optical isolators, each of which is made up of two polarizers, one Faraday element and one permanent magnet. It is apparent that this optical isolator has drawbacks in economical respects because the composing elements of which are too many as compared with the conventional polarization independent optical isolator.
As described above, it has been inevitable for the prior-art polarization independent optical isolators to perform precise and fine adjustments and compensation along the optical axis of the optical isolator for eliminating any defects resulting from angular errors in Faraday rotation angle. Further, another prior-art polarization independent optical isolator requires too many composing elements, such as two sets of polarization dependent optical isolators, thus having economical disadvantages.